Dynamoelectric machines utilizing permanent magnets to build up the magnetic field of rotors are known. Permanent magnets have been mounted in slots in the rotor of a synchronous motor or in the limb of the rotor. See, e.g., U.S. Pat. No. 3,840,763 to Baumann et al and U.S. Pat. No. 3,881,243 to Bannon (both incorporated herein by reference). Housing permanent magnets in a rotor can eliminate the need for exciting a field with direct current, however relatively large rotors may be required to utilize magnets of appropriate strength, and the magnets are subject to gradual demagnetization over time and more rapid demagnetization caused by forced reversals of the magnetic flux while the synchronous motor is in use.
While magnets can be protected against the demagnetizing field caused by alternating current, e.g., by providing leakage flux paths, however, such measures further increase the dimensions and weight of the machine, and add to its cost.
It has now been discovered that a rotor having a particular radial arrangement of permanent oriented ceramic magnets embedded or mounted in the plane of the rotor provides an improved magnetic field which does not reverse polarity or flux pattern of the magnets while rotating under demagnetizing conditions. Such magnetic rotors allow more efficient machine design, eliminate the necessity of constantly replacing magnets, conserve space within the dynamoelectric machine and reduce the cost associated with the rotor.